TEMPO

Tempo can be indicated in quarter notes per minute in written music, which frequently can he directly interpreted as beats per minute (bpm) in ACID. 

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Figure Bl.16.16 shows an example of RTPM in which the radical species is TEMPO (10), a stable nitroxide radical, while the triplet state is produced by photoexcitation of benzophenone (11) [45].

Figure Bl.16.16. TREPR spectrum of TEMPO radicals in 1,2-epoxypropane solution with benzophenone, 1 ps after 308 inn laser flash. Reprinted from [45],...

The tliree-line spectrum with a 15.6 G hyperfine reflects the interaction of the TEMPO radical with tire nitrogen nucleus (/ = 1) the benzophenone triplet caimot be observed because of its short relaxation times. The spectrum shows strong net emission with weak E/A multiplet polarization. Quantitative analysis of the spectrum was shown to match a theoretical model which described the size of the polarizations and their dependence on diffrision. 

Figure B2.4.8. Relaxation of two of tlie exchanging methyl groups in the TEMPO derivative in figure B2.4.7. The dotted lines show the relaxation of the two methyl signals after a non-selective inversion pulse (a typical experunent). The heavy solid line shows the recovery after the selective inversion of one of the methyl signals. The inverted signal (circles) recovers more quickly, under the combined influence of relaxation and exchange with the non-inverted peak. The signal that was not inverted (squares) shows a characteristic transient. The lines represent a non-linear least-squares fit to the data.

Systematic—Judgmental Sampling Combinations of the three primary approaches to sampling are also possible. One such combination is systematic-judgmental sampling, which is encountered in environmental studies when a spatial or tempo-... 

Fig. 6. Specific oxidation of the 6-hydroxyl of starch usiag bromide—hypochlorite and tetramethylpiperidine oxide (TEMPO).

Within each subcategoiy are any specially designated tempo-raiy procedures required for equipment operating in one of the four operating phases. These procedures are typically short lived due to a terminating condition or expiration date. They describe special tests or temporaiy changes in operating steps. These tempo-raiy procedures can be archived when they are not active. 

Nitro WjtnpoonrJ Cfabotiyl tempo and Conrlidon Yield 1%) Ptoflace l.r) Ref. 

High concentrations of SO, can produce tempo-rai y breathing difficulties in asthmatic children and in adults who are active outdoors. Sulfur dioxide also can directly damage plants and has been shown to decrease crop yields. In addition, sulfur oxides can be converted to sulfuric acid and lead to acid rain. Acid rain can harm ecosystems by increasing the acidity of soils as well as surface waters such as rivers, lakes, and streams. Sulfur dioxide levels fell, on average, by 39 percent between 1989 and 1998. 

Transmission line faults are predominantly tempo-rai"y, and automatic reclosing is a necessaiy complement to the protective relaying function. The reclose time must be greater than the time required to dissipate the arc products associated with the fault. This varies with the system voltage and ranges from 15—2(1 cycles at 138 kV to 30 cycles for the 800 kV systems. Automatic reclosing requires that proper safety and operating interlocks arc provided. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

The identification of both phenylethyl and 1-phenyl-1,2,3,4-lelrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMP provides the most compelling evidence for the Mayo mechanism.316 The 1-phenyl-1.2,3,4-tetrahydronaphthalenvl end group is also seen amongst other products in the TEMPO mediated polymerization of styrene,317318 However, the mechanism of formation of radicals 96 in this case involves reaction of the nitroxide with the Diels-AIder dimer (Scheme 3.63). The mechanism of nitroxide mediated polymerization is discussed further in Section 9.3.6. 

The reaction between nitroxides and carbon-centered radicals occurs at near (but not at) diffusion controlled rates. Rate constants and Arrhenius parameters for coupling of nitroxides and various carbon-centered radicals have been determined.508 311 The rate constants (20 °C) for the reaction of TEMPO with primary, secondary and tertiary alkyl and benzyl radicals are 1.2, 1.0, 0.8 and 0.5x109 M 1 s 1 respectively. The corresponding rate constants for reaction of 115 are slightly higher. If due allowance is made for the afore-mentioned sensitivity to radical structure510 and some dependence on reaction conditions,511 the reaction can be applied as a clock reaction to estimate rate constants for reactions between carbon-centered radicals and monomers504 506"07312 or other substrates.20... 

Examples of radicals which are reported to meet these criteria are diphenylpicrylhydrazyl [DPPH, (22)], Koelsch radical (26), nitroxides [e.g. TEMPO (23), Fremy s Salt (24)], triphenylmethyl (25), galvinoxyl (27), and verdazyl radicals [e.g. triphenylverdazyl (28)]. These reagents have seen practical application in a number of contexts. They have been widely utilized in the determination of initiator efficiency (Section 3.3.1.1.3) and in mechanistic investigations (Section 3.5.2). 

The majority of polymers formed by living radical polymerization (NMP, ATRP, RAFT) will possess labile functionality at chain ends. Recent studies have examined the thermal stability of polystyrene produced by NMP with TEMPO (Scheme 8.3),2021 ATRP and RAFT (Scheme 8.4).22 In each case, the end groups... 

The use of the disulfide (13), which can dissociate thermally to give a sulfur analog of TEMPO (Section 9.3.6.1), has also been explored for controlling S polymerization though poor results were obtained.40... 

Catala and coworkers167JuiS made the discovery that the rate of TEMPO-mediated polymerization of S is independent of the concentration of the alkoxyamine. This initially surprising result was soon confirmed by others.23 69 Gretza and Matyjaszewski169 showed that the rate of NMP is controlled by the rate of thermal initiation. With faster decomposing alkoxyamines (those based on the open-chain nitroxides) at lower polymerization temperatures, the rate of thermal initiation is lower such that the rate of polymerization becomes dependent on the alkoxyamine concentration, Irrespective of whether the alkoxyamine initiator is preformed or formed in situ, low dispersities require that the alkoxyamine initiator should have a short lifetime. The rate of initiation should be as fast as or faster than propagation under the polymerization conditions and lifetimes of the alkoxyamine initiators should be as short as or shorter than individual polymeric alkoxyamines. 

Various methods have been used to form low molecular weight alkoxyamine initiators for NMP. Most involve forming an appropriate carbon-centered radical in the presence of a nitroxide. Initiators that generate carbon-ccntcrcd radicals may be thermally decomposed in the presence of a nitroxide. For example, alkoxyamine 100 is formed by decomposition of AIBN in the presence of TEMPO (Scheme 9.19). 1,1 Carbon-centered radicals may also be generated photochemically. 70... 

ATRP catalysts may be used to generate radicals and thus alkoxyamines can be produced from alkyl halides in high yield (Scheme 9.21).174 The alkoxyaminc 102 was obtained in 92% yield 174 whereas reaction of TEMPO with PMMA under ATRP conditions is reported to provide a macromonomer (Section 9.7.2.1). 

The thermal decomposition of the phenylelhyl alkoxyamine with TEMPO and the fraction of living ends in TEMPO-mediated S polymerization has been studied by Priddy and coworkers.143 179 They concluded that to achieve >90% living ends conversions and/or nitroxide concentrations should be chosen to give V/ less than 10000.143 However, disproportionation or elimination is most important during polymerizations of methacrylates and accounts for NMP being less successful with... 

NMP has mainly been used for S polymerization (9.3.6.5.1) and, to a lesser extent, acrylate (9.3.6.5.2) polymerization. The early and much current work has focused on the use of TEMPO and derivatives. The open chain nitroxides 86-91 ( fable 9.3) provide broader though still restricted utility. Some of the previously difficult monomers that have recently been tackled successfully include HEA,196 DM AM197 and A A198 199 with nitroxide 89. 

NMP with acrylates and acrylamides with TEMPO provides only very low conversions. Very low limiting conversions and broad dispersities were reported.2 Better results were obtained with DTBN (83),111 151 imidazoline (61-64)I3S and isoindoline (59) nitroxides.111 However, limiting conversions were still observed. The self-regulation provided in S polymerization by thermal initiation is absent and, as a consequence, polymerization proceeds until inhibited by the buildup of nitroxide. The final product is an alkoxyamine and NMP can be continued... 

Of the major methods for living radical polymerization, NMP appears the most successful for polymerization of the diene monomers. There are a number of reports on the use of NMP of diene monomers (B, I) with TEMPO,188,1103 861 4, cw and other nitroxides.127 High reaction temperatures (120-135 °C) were employed in all cases. The ratio of 1,2- 1,4-cis 1,4-trans structures obtained is similar to that observed in conventional radical polymerization (Section 4.3.2). 

The early attempts at NMP of S in emulsion used TEMPO and related nitroxides and needed to be carried out at high temperatures (100-130 °C) necessitating a pressure reactor. Problems with colloidal stability and molecular weight control and limiting conversions were reported.215 217... 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

NMP in miniemulsion has been more successful. In miniemulsion polymerization nuclealion lakes place directly in the monomer droplets that become the polymer particles. Particle sizes are small (<100 nm). Most w ork has used TEMPO and high reaction temperatures (120-140 °C) with S or BA as monomer. 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

Addition of TEMPO post-polymerization to a methacrylate polymerization provides an unsaturated chain end (Scheme 9.52)i07 sw presumably by disproportionation of the PMMA propagating radical with the nitroxide. For polymers based on monosubstituted monomers (PS,1 0" PBA59,[Pg.534]

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